Summer Rains Increase Risk of Human Viruses in Groundwater

By Kelly A. Reynolds, MSPH, PhD

Gazette note: Below is a truncated version of an excellent article from the June 2017 issue of Water Conditioning & Purification. Dr. Kelly A. Reynolds is a widely recognized authority on water quality issues and especially microbial contamination. The increasing frequency of waterborne disease described in the article explains the growing popularity of point of use and point of entry home treatments like ultraviolet disinfection.

Just as the weather constantly varies, the quality of source water is also ever-changing. Increased rainfall in spring and summer months creates additional challenges to municipal water suppliers and private well owners as water moving over the land and through the soil accumulates added contaminants capable of causing human disease.

Heavy rainfall associated with waterborne disease

Surveys of extreme precipitation events indicate (rainfall more than two inches a day) and waterborne disease outbreaks (WBDO) in the US are strongly correlated. Retrospective comparison of 548 outbreaks documented by US EPA and precipitation data from the National Climatic Data Center from 1948 to 1994 showed that 68 percent of WBDOs were preceded by extreme precipitation events. Surface water was the most likely to be contaminated and result in an outbreak during the same month as the rainfall event but groundwater outbreaks lagged by about two months.

Twenty-four years ago in late March, the largest documented waterborne outbreak in US history occurred in Milwaukee, WI. Before identifying the problem, residents consumed contaminated water for over two weeks. Ultimately, more than 400,000 people were sickened with diarrhea and over 100 died. Cryptosporidium, a protozoan pathogen, caused the outbreak and may have been introduced due to increased precipitation and the presence of nearby cattle farms. Crypto has been found in 64 percent of manure samples from a sampling of 50 livestock farms. Following rain and land runoff, Crypto from nearby farms is readily transported to surface supplies, where associated increases in turbidity further tax treatment works.

Groundwater risks

Surface water risks are somewhat expected and municipalities have treatment tools, including the use of flocculants, filtration and disinfectants to settle out, filter and inactivate harmful microbes. While federal regulations mandate treatment of surface water, utilities accessing groundwater are not necessarily required to treat. Thus, less obvious and less controlled are groundwater contamination events. The greatest concern with seasonal groundwater contamination are human viruses. Viruses, unlike larger bacteria and protozoa, easily navigate the tortuous path from land surface to underground aquifers. Storms, however, can lead to sewer overflows and contamination of groundwater wells with a variety of microbial hazards.

Recently Minnesota and Wisconsin state health departments announced evidence of disease-causing microbes in a high percentage of drinking-water wells. In Minnesota, eight percent of a collection of 478 samples and 37 percent of the 82 public water systems with a groundwater well supply tested positive for human viruses. Eleven percent were positive for Salmonella bacteria. Less is known about household well water supplies. An estimated 34 million households in the US are served by private wells. One Wisconsin study found that out of 50 wells from seven hydrogeologic districts, eight percent were positive for human viruses, including hepatitis A virus, rotavirus, and noroviruses. With summer being Wisconsin’s rainy season, concern this time of year is especially heightened.

Most private and public groundwater supplies are not filtered or disinfected. The presence of low levels of human virus genomes in groundwater is common and has been associated with a 30 percent increase in gastrointestinal illness. Up to 63 percent of gastrointestinal illnesses in children were attributed to these tap-waterborne viruses.